View Compilation Error

0_0_33272509_7894.cpp:19:36: error: 'operator*=' function uses 'auto' type specifier without trailing return type
     auto operator*=(const Matrix &z) {
                                    ^
0_0_33272509_7894.cpp:19:36: note: deduced return type only available with -std=c++1y or -std=gnu++1y